China's internet users have been cut off from accessing all foreign websites for around an hour in an unexplained incident that sparked speculation the country's censorship system was being tested or further tightened.

The "great firewall" already blocks many sites hosted from other countries, but users in Beijing, Shanghai and other parts of China reported that they could not reach any foreign sites whatsoever on Thursday morning – although it was not clear whether the problems were universal.

Meanwhile, users abroad and in Hong Kong – which is part of China but not subject to Beijing's net censorship – said they were unable to reach any sites on the Chinese mainland.

Some believed it was purely a technical failure, with several suggesting that Wednesday's massive earthquakes had hit an undersea cable, disrupting services. In 2007, a tremor hit a major cable and dramatically slowed access to overseas sites for months.